This invention relates generally to the field of high pressure processing of materials. More particularly, this invention relates to processes for compaction and consolidation of materials using isostatic application of high pressures in combination with application of high temperatures for short periods of time, and, further relates to novel conditions of matter formed thereby.
An important technique in modern materials science and metallurgy is the application of heat and isostatic pressure to a material in a process known as "hot isostatic pressing" or "HIP". HIP is a very versatile tool for the production or improvement of materials in a variety of ways. HIP is frequently used to eliminate porosity from a material thereby producing a fully dense material with improved properties. HIP is also used to bond together dissimilar materials which are not otherwise conveniently joined into a single, integrated part. Also, HIP is commonly used to compress powders into a fully dense, solid part.
We will use the term "workpiece" to denote any material, or combination of materials, to be HIP processed. As noted above, such workpieces could be a single solid material requiring densification, dissimilar materials to be bonded together, or powders requiring compaction. In the case of powders, the powders may be held in the desired final shape during HIP processing by an appropriate mold, by preliminary mechanical pressing, by the use of binding agents, or by several other techniques and combinations of techniques well known in the field. For economy of language, "workpiece" as used herein will encompass any such starting material for HIP.
Conventional HIP is commonly characterized as a "creep" process. Pressure is applied to force into tight union the various surfaces to be bonded. Heat is applied to increase the molecular motion of the proximate surfaces, leading to diffusion of one surface into another. This mass diffusion resulting from molecular inter-penetration of surfaces held together by pressure leads to fully dense, fully chemical bonded materials typical of HIP processing. Typically, conventional HIP processing requires hours at elevated temperature and pressure for adequate mass diffusion and densification to occur; that is, for the surfaces to "creep" together.
A significant drawback to conventional HIP processing is the length of time (typically hours) the workpiece must spend at elevated temperatures and pressures. Thus, workpieces typically must occupy the HIP press for many hours, reducing throughput and increasing processing costs per part. The typical response to this economic drawback of conventional HIP has been to limit HIP to expensive parts in which HIP processing costs are tolerable. Thus, the beneficial properties in workpieces created by HIP processing are not commercially available for many workpieces. An alternative response to long processing times for conventional HIP has been to process parts in large quantities in a large HIP press. This suffers from the twin disadvantages of requiring even more massive (and, hence, expensive) equipment, and requiring an accumulation of workpieces before HIP processing will commence (running counter to the overriding modern thrust for "just-in-time" processes to minimize inventory and work-in-progress manufacturing costs).
Many approaches have been taken to reducing the time of HIP processing and, hopefully, reducing thereby the processing cost per workpiece. Previous work of ours (U.S. Pat. No. 4,856,311) is one approach to increasing the speed of HIP processing. A container holding workpieces is rapidly pressurized with gas to achieve HIP processing results in a much reduced period of time. This approach markedly reduces the time each workpiece resides in the pressure vessel. In this approach the workpieces may typically be preheated prior to introduction into the HIP press, further minimizing the residence time in the pressure vessel.
Another approach to rapid processing is "explosive forming". Explosive forming is descriptively named in that workpieces are forced together by an "implosion" of the workpiece caused by a surrounding "explosion" of a conventional chemical explosive. The resulting process involves both high pressures from the explosion and high temperatures formed by the energy of the explosion, as released directly and generated within the workpiece by friction between surfaces rapidly compressed together. Explosive forming is not suited for many workpieces due to the extreme conditions typically generated during the explosive processing, and the difficulty in controlling the process parameters of time, temperature and pressure.
In contrast to conventional HIP processing, conventional forging can typically shape and densify materials very rapidly. Typical forging processes use a workpiece placed into a suitable mold or chamber and heated. While held in place by mechanical reaction forces generated by the mold, high pressures are applied to the workpiece, typically by a mechanical force or ram (or, in some cases, by a blacksmith's hammer). Such forces are not truly isostatic and have significant directional components, generating nonuniform pressures along different directions of the workpiece. Many researchers have recognized the disadvantages of this non-isostatic application of forces and have attempted to mitigate the application of mechanical ram forces in a variety of ways. Several researchers have used various low-yield strength solids (typically powders) surrounding the workpiece in an attempt to distribute the force of the mechanical ram more evenly about the workpiece (i.e. U.S. Pat. Nos. 3,356,496; 4,389,362; 4,431,605; 4,539,175). The work of Inoue (U.S. Pat. No. 4,414,028) uses six separately controlled mechanical rams in addition to a solid surrounding pressure transmitting medium as an approach to achieving more isostatic pressure application to the workpiece.
All of these "pseudo-isostatic" processes suffer from one or more disadvantages. Shearing forces of the pressure transmitting medium do not allow uniform pressure to be applied to the workpiece, resulting in possible workpiece distortion. The pressure transmitting medium may not flow adequately around workpieces with complex shapes, leading to shape-dependent processing. The pressure transmitting medium itself may be damaged by the high process temperatures and pressures. The process of removal of the pressure transmitting medium, detaching it adequately from the workpiece, and reprocessing it for reuse (sometimes with heating of the medium as in U.S. Pat. No. 4,539,175 or its commercial tradename process "Ceracon") are additional processing steps.
Rapid HIP processing can be achieved by generating pressures, not with mechanical rams, but by thermal expansion of gases (U.S. Pat. No. 4,856,311; 4,942,750 and Robert M. Conaway, "Cost-Effective Isostatic Forging" in Advanced Materials and Processes, June 1989). These processes employ encapsulation of a workpiece in a suitable container (if required), typically followed by heating of the workpiece outside the pressure-containment vessel, introduction of the workpiece into the pressure-containment vessel and rapid application of pressure achieved through thermal expansion of gases. The resulting apparatus and process achieve both genuine isostatic processing of HIP and a significant reduction in processing time in comparison with conventional HIP. "Fast-HIP", "Quick-HIP" or "gas forging" are shorthand terms which have been used to denote this improvement on conventional HIP processing of workpieces, using thermal expansion of gases to generate elevated pressures: "fast-HIP" emphasizing the achievement of fast processing times without sacrificing the advantages of HIP, while "gas forging" emphasizes the use of gas to replace the mechanical ram of conventional forging processes. Gas forging succeeds in reducing the time a workpiece occupies the pressure chamber, thereby increasing throughput for expensive pressure processing equipment. In addition, the use of gas as the pressure transmitting medium eliminates the problems noted above with shearing forces in the pressure transmitting medium; with difficulties of completely surrounding workpieces having complex shapes; with possible damage to a solid pressure transmitting medium during processing; and with the reprocessing (and reheating) of the solid pressure transmitting medium for reuse.
Previous HIP and forging processes are typically carried out with the heating of the workpiece prior to (or simultaneously with) the application of pressure. This is the case in conventional forging in which a mechanical ram typically impacts a hot workpiece, and in the pseudoisostatic forging processes noted above. Such heating prior to application of pressure is also the case in fast-HIP, as described in the cited patents, in which heating of the workpiece occurs typically outside the pressure-containment vessel.
However, elevated temperatures can have serious detrimental effects on many workpieces. Typically, four general classes of problems occur when elevated temperatures are employed: 1) In two-phase systems (typically, fiber-containing or composite materials), high temperatures may reduce the strength of the bonding between phases, thereby reducing the overall strength of the material. 2) Certain materials are metastable and, in spite of this, very useful in many applications. (Rapidly solidified materials are an example). The metastability of such materials is often destroyed by lengthy exposures to elevated temperatures, causing the material to revert to its thermodynamically stable (and less useful) form. 3) Elevated temperatures can cause structural changes in the atomic or molecular configuration of the material, affecting thereby its properties. Typically, exposure to elevated temperature can lead to the growth of a larger grain structure within the material, detrimentally affecting the properties of the material. 4) Elevated temperatures may cause chemical reactions to occur, or decomposition of component molecules, changing thereby the chemical composition (stoichiometry) of the workpiece.
The present invention describes a method in which the time required for the workpiece to be at elevated temperatures is reduced, sometimes dramatically, while retaining the advantages of HIP processing. The elimination of deleterious effects of high temperatures leads to novel processing possibilities, and can result in novel materials not heretofore produced by other processes. The present approach should be contrasted (for example) with that of Lueth (U.S. Pat. No. 4,431,605) in which temperatures of the workpiece at or near the liquid phase temperature are used for the purpose of reducing the pressure required for HIP processing. It is an important goal of the present invention to show the advantages of minimizing, not the processing pressure, but the application of high processing temperatures to the workpiece.